1. Field of the Invention
This invention relates to a rear-projection screen comprised of a Fresnel lens and a light-diffusing member such as a lenticular lens sheet.
2. Description of the Related Art
As methods for displaying large-screen pictures, a method is known in which an optical image is enlarged and projected onto a rear-projection screen from a CRT (cathode-ray tube), a liquid crystal panel or the like through a projection lens.
FIG. 5 shows an example of the constitution commonly employed in display devices capable of forming pictures by such a method. In the display device shown in FIG. 5, optical images sent from CRTS 1 respectively corresponding R (red), G (green) and B (blue) are enlarged through projection lenses 2 and formed as an image onto the surface of a two-sheet type rear-projection screen comprised of a Fresnel lens 3 and a lenticular lens sheet 4. Here, the Fresnel lens 3 has a function of directing the incident light substantially toward the position of a viewer and the lenticular lens sheet 4 has a function of dispersing the light emerging from the Fresnel lens 3, at given angles in the horizontal and vertical directions and in a suitable proportion of distribution to expand a visual angle to the angle of a given scope. As another rear-projection screen known in the art, the lenticular lens sheet 4 in the rear-projection screen 5 shown in FIG. 5 may be replaced with a light-diffusing member comprising a flat plate or flat sheet in which light-diffusing fine particles have been dispersed.
In such display devices, however, the occurrence of color shift and a lowering of white uniformity come into question, the former being a change in color tones of images on a screen that may occur when the position at which the screen is viewed is changed in the horizontal direction and the latter being a difference in color tones at some positions on the screen being viewed, caused by the convergent angles .epsilon. of the light rays projected from the respective R, G and B CRTs 1.
Now, in order to decrease the color shift and raise the level of the white uniformity, it is hitherto known to use, as shown in FIG. 4, a double-sided lenticular lens sheet 30 comprised of an entrance lens 31 comprising a cylindrical lens formed on the entrance surface, an exit lens 32 comprising a cylindrical lens also formed on the exit surface, and a light absorbing layer 33 (what is called a black stripe) formed at the light non-convergent part of the exit surface. In such a double-sided lenticular lens sheet 30, various attempts to improve the shape or optical axis shift of each lens element constituting the entrance lens 31 or exit lens 32 have been proposed in order to decrease the color shift while ensuring a wide visual field angle and to raise the level of the white uniformity. Besides the attempt to specify the shape and so forth of each lens element, it is also attempted to uniformly disperse light light-diffusing fine particles throughout the lenticular lens sheet so that the horizontal visual field angle can be ensured chiefly by its lenticular lens and the vertical visual field angle can be ensured by the light diffusion properties of the fine particles. Since the resolution may become lower because of light scattering caused by the fine particles when such light-diffusing fine particles are used to ensure the vertical visual field angle, it is also proposed to form minute concaves and convexes on the surface of the exit lens by various methods.
With such constitution, however, it has been difficult to well improve the properties of the screen if elementary lenses of the lenticular lens sheet are arranged in a small pitch in order to obtain images with a high resolution. Namely, if elementary lenses of the lenticular lens sheet are arranged in a small pitch in order to obtain images with a high resolution, the horizontal visual field angle becomes small when the thickness of lenses is unchanged. Hence, it becomes necessary to make the thickness of lenses smaller in order to ensure the desired horizontal visual field angle. For example, if it is intended in a conventional lenticular lens sheet to obtain a horizontal half visual field angle of 37.degree. in a lens pitch of 0.6 mm, it becomes necessary to form the lenticular lens sheet in a thickness of as small as 0.78 mm. It, however, is difficult in itself to produce lenticular lens sheets with a thickness of as small as 0.78 mm, by extrusion which is a process presently available for efficient mass production of lenticular lens sheets, and there is a problem that the resulting products tend to break. Lenticular lens sheets can be stably produced by extrusion if they are formed in a thickness of 0.9 mm or more. Such a thickness, however, can not be enough to achieve the horizontal half visual field angle of 37.degree. . Meanwhile, if a lenticular lens sheet is made to have a thickness large enough to be stably produced and the light-diffusing fine particles are used to ensure the horizontal visual field angle, the color shift can be better prevented, but the light diffusion inside the lenticular lens sheet may increase to cause an increase in the amount of the light entering into its light absorbing layer, so that another problem of a decrease in the amount of emergent light may occur.
Accordingly, the present inventors have proposed, as shown in FIG. 3, a lenticular lens sheet comprising a double-sided lenticular lens having a light absorbing layer 23 on its exit side, wherein the double-sided lenticular lens is formed of a double-layered lens comprised of an entrance lens layer 21 and an exit lens layer 22, the light-diffusing fine particles are dispersed in the exit lens layer 22, and also the exit lens layer 22 is formed in a thickness smaller than the entrance lens layer 21 (Japanese Patent Application Laid-open No. 5-61120). In such a double-layered lenticular lens sheet 20, the light-diffusing fine particles are dispersed in a high concentration in the thin exit lens layer 22, and hence the light can be made to well diffuse in the exit lens layer to thereby expand the visual field angle. Moreover, the diffused light can be prevented from entering into the light absorbing layer 23. Thus, it becomes possible to increase the utilization of light, make pictures bright, and make the visual field angle larger and the lens pitch smaller to enhance the resolution. In addition, since the light diffusion elements are concentrated on the exit surface of the lenticular lens, it becomes possible to decrease the moire caused by the overlap of a dark line ascribable to a rise face of the Fresnel lens (the face connecting a land and a valley of the lens face) with the light absorbing layer of the lenticular lens.
Even in the lenticular lens sheet disclosed in the above Japanese Patent Application Laid-open No. 5-61120, however, when it is intended to obtain much brighter pictures, the light diffusion properties of the lenticular lens sheet must be made smaller than usual by decreasing the amount of the light-diffusing fine particles or making smaller the thickness of the exit lens layer in which the light-diffusing fine particles have been dispersed. In such a case, the dark line ascribable to the rise face of the Fresnel lens can not be made substantially thin by well diffusing the light in the exit lens layer and hence the moire can not be well decreased. Such a problem has remained unsettled.
In recent years, there is a demand for making projection devices as compact as possible. As the projection distance is made shorter according to such a demand, the field angle (or angle of view) 2.theta. as shown in FIG. 11 becomes larger, which is the angle formed by light rays shed from a light source 1 of a CRT or the like toward the edges of a rear-projection screen 5. Then, in general, the intensity of the light made incident on the edge portions of the rear-projection screen 5 becomes weak in proportion to cos.sup.4 .theta.. Light loss due to reflection on the Fresnel lens 3 also increases as the field angle becomes larger. Hence, Iris common for the intensity of the light emerging from the Fresnel lens 3 to become smaller at the border (herein meant to be the boundary of a surface, including its inside area with a certain breadth) of the screen as the projection distance is made shorter. For this reason, the light emerging from the border of the lenticular lens sheet 4 toward a viewer also becomes weak to bring about the problem that the border of the screen, in particular, the four corners thereof give a dark feeling. More specifically, the state of diffusion of the light emerging from the center and border of the lenticular lens sheet 4 can be expressed as shown in FIG. 11. As shown in the drawing, when an intensity of the light emerging in the direction where the diffused light intensity is greatest at the border of the screen is represented by L.sub.2 and an intensity of the light emerging toward the position at which the viewer usually views the screen (the position 3 to 5 m distant from the center of the lenticular lens sheet 4) is represented by L.sub.1, the L.sub.1 becomes weak as the projection distance is made shorter. The problem that the screen, in particular, the four corners thereof give a dark feeling when the projection distance is shortened is similarly seen also in the case when the lenticular lens sheet disclosed in the above Japanese Patent Application Laid-open No. 5-51120 is used.
As the rear-projection screen used when a bright picture is formed on a large screen in a high resolution, those making use of the double-layered lenticular lens sheet as described above are preferred. Besides these, when a relatively small-screen picture is formed, a rear-projection screen comprised of a combination of a flat plate or flat sheet in which light-diffusing fine particles have been dispersed with a Fresnel lens and a rear-projection screen comprised of a combination of a single-layer lenticular lens sheet with a Fresnel lens are still put into use at present, without use of the double-layered lenticular lens sheet. Like the rear-projection screen making use of the double-layered lenticular lens sheet described above, these rear-projection screens also have the problem that the border of the screen, in particular, the four corners thereof give a dark feeling when the projection distance is made shorter according to the recent demand for making projection devices as compact as possible. In the rear-projection screen making use of the single-layer lenticular lens having a light absorbing layer on its exit lens layer, there is also the problem that the moire becomes conspicuous at the border of the screen.